Anita Mendoza Presents with a Large Failing #30 MOD Amalgam

Essay

Anita Mendoza presents with a large failing #30 MOD amalgam with distal lingual cusp fracture. She says she hated to have the tooth "worked on again because it's the only tooth that never gets numb no matter what they try." It is determined that a crown is the appropriate treatment for #30. She is assured that the tooth will be numb before treatment. IA, lingual and buccal nerve blocks are administered with a total of 1.5 cartridges of lidocaine with 1:1,000,000 epinephrine. She reports the "tooth and all around it feel numb," however when a pulp tester is applied to #30 she jumps with discomfort. Pulp testing on tooth number 29 and 31 are negative. The IA injection site is carefully re-evaluated and an IA injection is repeated with a second cartridge of the same drugs. An identical response to the pulp tester follows.
-What are the possible causes for inadequate anesthesia of tooth #30?